*excuse me for the long tech answer, but since i'm with agtech startup that helped many agribusinesses to make vr planting, i believe this reply could be helpful.

There is no one-size-fits-all answer.

Please check this out How to create Variable Rate Seeding (Planting) Maps, we've described popular solutions with visuals.

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Consider taking into analysis following layers:
- Yield data(clean/calibrate if needed(outliers, bad numbers or several harvesters worked), fill the gaps with synthetic yield map, if yield data covers field only partially)
- Soil Sampling/Scanning
- Historical Imagery and Field Potential based on them (0 clicks, created totally automatically)
- Topography (revers slope attribute is a good option (reverse means you just put minus -1 weight to it)
- Soil Brightness (brighter soils usually lower OM, depends a lot on your soils)

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Play with the results in iterative manner via multi-layer zoning module, interactive demo.
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Is your field yield is stable from year to year? You can check it with some tools: 
- automated field potential maps
- heterogeneity factor
- In time variation maps (dark blue means not stable from year to year)

If it's stable, then Field potential is your friend, if not - carefully choose layers for VR planting.

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Clustering of zones: 

Do you do various zones per operation or use the same for planting, fertilizer? 

If the same - take a look into spatially localized zones (in simple words - zone of one color is located only in one place, no polygons of the same color/rate located across the field). This is not as "cold" as grids (since based on layers you choose, and no so complex to manage as multi-polygons zones).

Soil sampling zones IMO are much more helpful for fertilizer application (then you use equations or just a simple zoning based on soil attribute).

Grid is suboptimal, consider using spatial clusters (see above), that could be stable and easier to manage and reflect your field much better that "cold" grids.
Here is a use-case how it can be scaled (1m acres farm).

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Feel free create free trial account, will be enough for this field, play with creating of optimal maps, you can also export vra maps created (shp, iso or send to John Deere), lidar topography, images (history from 1988). 
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Cheers